In various circuitry, amplifiers are commonly used to boost an amplitude of an incoming signal to a desired level. For example, various amplifiers may be present in a given system to boost signal strength of incoming signals to provide them at a desired level for further processing.
In wireless systems such as cellular handsets, mobile internet devices, wireless personal digital systems (PDAs) and so forth, typically a power amplifier (PA) is present to receive a radio frequency (RF) signal modulated as desired for a given communication protocol and amplify this signal for transmission via an antenna of the device. Typically, a power amplifier can amplify both current and voltage of an incoming signal to provide the signal at a desired level.
Different requirements may exist in different communication protocols. Many communication systems have various requirements for a handset to achieve with relation to power, efficiency, and linearity over varying signal levels. For example, a variety of communication systems, including enhanced data rates for GSM evolution (EDGE), long term evolution (LTE/4G), WiFi in accordance with an IEEE 802.11 standard, worldwide interoperability for microwave access (WiMax), code division multiple access (CDMA), and wideband-code division multiple access (W-CDMA) all have modulation schemes that require a linear signal path.
In an amplifier stage of such a PA, if a phase shift through the stage is a function of the amplitude of the input signal, then that amplifier has phase distortion (a.k.a. amplitude modulation-to-phase modulation (AM-to-PM) distortion). AM-to-PM distortion is a non-linear process which degrades the amplifier's overall linearity. AM-to-PM distortion can exist with or without amplitude (a.k.a. AM-to-AM) distortion. Together the two non-linear processes characterize the non-linear behavior that is relevant in ideally linear communications circuits. These non-linear processes cause spectral splatter or leakage of signal energy from a desired channel to nearby channels. In many systems, this spectral splatter is measured by the adjacent channel power ratio (ACPR) or the adjacent channel leakage ratio (ACLR).
Current PAs are typically formed using a gallium arsenide (GaAs) process with bipolar devices that do not suffer significant AM-to-PM distortion. However, in the case of highly efficient complementary metal oxide semiconductor (CMOS) power amplifiers, AM-to-PM distortion can cause a significant linearity problem. AM-to-AM linearization techniques exist for CMOS devices, but such techniques either do not address AM-to-PM distortion, or they are rendered less effective because of AM-to-PM distortion. As a result, an amplifier such as a power amplifier that is formed using CMOS devices can have linearity issues. Alternatively, the CMOS power amplifier can be made to operate in a different mode (a.k.a. Class A) that improves the overall linearity but reduces the efficiency.